1. Field of the Invention
The present invention relates to an apparatus for removing a pulsive noise in an FM receiver. More specifically, the present invention relates to an apparatus for removing effectively a pulsive noise included in a received signal in an FM receiver.
2. Description of the Prior Art
It has been well known that a pulsive noise, such as an ignition noise caused by an automobile, interferes with normal reception by an FM receiver. Since such a pulsive noise tends to phase modulate the FM signal, it cannot be removed by an amplitude limiter and is liable to be detected by a detector and to be transferred to the subsequent stage. Accordingly, it is necessary that such a pulsive noise be removed in a signal transmission path in the stage subsequent to the detector.
Hitherto, two method for removing such a pulsive noise have been proposed and put in practical use. One method is to provide a gating circuit between the detector and a stereo multiplex circuit such that the gating circuit is opened on the occasion of occurrence of a noise, thereby to remove the noise. The other method is to provide gating circuits in the two output signal lines for left and right signals, respectively, from a stereo multiplex circuit, such that these gating circuits are opened on the occasion of occurrence of the noise, thereby to remove the noise. These two methods each have advantages, but nevertheless have disadvantages that a pulse cannot be completely removed.
Accordingly, the present invention is directed to an improvement in a prior art method wherein a gating circuit is provided between a detector and a stereo multiplex circuit in an attempt to remove noise in a thorough manner.
Before entering into description of the inventive noise removing apparatus, a brief description will be made of a typical prior art noise removing apparatus with reference to FIG. 1. Referring to FIG. 1, the reference numeral 1 denotes an input terminal where a detected output signal is applied, the reference numeral 2 denotes an input transistor, the reference numeral 3 denotes a gate adapted to be kept open for a predetermined time period after receipt of a pulsive noise, the reference numeral 4 denotes a holding capacitor, the reference numeral 5 denotes an output transistor, and reference numeral 6 denotes an output terminal. The gate 3 is closed while no pulsive noise exists, so that the detected output signal is transferred from the input terminal 1 through the input transistor 2, the gate 3 and the output transistor 5 to the output terminal 6. At that time, the holding capacitor 4 is kept charged by the detected output signal. If and when a pulsive noise is received, a noise detector (not shown) becomes operative to open the gate 3. Since the noise detector has been adapted to keep the gate 3 open for a predetermined time period upon detection of the noise, the output transistor 5 is kept supplied for that time period from the holding capacitor 4 with a signal of the signal level immediately before the gate 3 is opened. Thus, the pulsive noise is effectively removed and is not transferred to the subsequent stages.
However, such a noise removing apparatus has a disadvantage in that even the pilot signal is interrupted during the time period when the gate is opened. If the pilot signal is interrupted for a predetermined time period, the switching signal for the stereo multiplex circuit is not obtained in the subsequent stage. As a result, the stereo multiplex circuit is switched from the stereo operation to monaural operation. If and when the gate is closed, the stereo multiplex circuit is switched back from monaural operation to stereo operation. Such a noise removing apparatus suffers from the shortcomings that distortion and noise are caused at the time of such switching between the stereo and monaural operations. Another disadvantage is that while the gate is opened the output signal is retained in the pilot signal voltage and that state is demodulated by the stereo multiplex circuit, which causes a noise.
In order to solve the above described disadvantages, provision may be made wherein while the gate is opened the pilot signal is applied from a separate circuit. To that end, it has been proposed that an LC parallel resonance circuit is provided in series with the holding capacitor, whereby a pseudo pilot signal is supplied from the LC parallel resonance circuit during the time period when the gate is opened.